One popular view of Venus’ climate history describes a world that has spent much of its life with surface liquid water, plate tectonics, and a stable temperate climate. Part of the basis for this optimistic scenario is the high deuterium to hydrogen ratio from the Pioneer Venus mission that was interpreted to imply Venus had a shallow ocean’s worth of water throughout much of its history. Another view is that Venus had a long lived (~100 million year) primordial magma ocean with a CO2 and steam atmosphere. Venus’ long lived steam atmosphere would sufficient time to dissociate most of the water vapor, allow significant hydrogen escape and oxidize the magma ocean. A third scenario is that Venus had surface water and habitable conditions early in its history for a short period of time (<1 Gyr), but that a moist/runaway greenhouse took effect because of a gradually warming sun, leaving the planet desiccated ever since. Using a general circulation model we demonstrate the viability of the first scenario using the few observational constraints available. We further speculate that Large Igneous Provinces and the global resurfacing 100s of millions of years ago played key roles in ending the clement period in its history and presenting the Venus we see today. The results have implications for what astronomers term “the habitable zone,” and if Venus-like exoplanets exist with clement conditions akin to modern Earth we propose to place them in what we term the “optimistic Venus zone.”

Foreshocks provide valuable information on the nucleation process and mechanism of impending earthquakes. In this study, we utilized the Match&Locate method to build a high-precision foreshock catalog for the July 2019 Mw 6.4 Ridgecrest, California earthquake. The Mw 6.4 mainshock was preceded by 40 foreshocks within ~2 hours (on July 4, 2017 from 15:35:29 to 17:32:52, UTC). Their spatiotemporal distribution reveals a complex seismogenic structure consisting of multiple fault strands, which were connected as a throughgoing fault by later foreshocks and eventually accommodated the 2019 Mw 6.4 mainshock. To better understand the nucleation mechanism, we conducted a series of analysis for the foreshocks including repeating earthquake identification, rupture directivity inversion, and Coulomb stress change estimation. We identified a pair of small earthquakes with close magnitude, high waveform similarity, and high cross-spectral coherence at the early nucleation stage. However, we cannot confirm if they are repeating earthquakes due to their low magnitude and insufficient sampling rate. Thus, the initial nucleartion mechanism is unclear to us. Following the largest ML 4.0 foreshock, we found the majority of its aftershocks and the Mw 6.4 mainshock occurred within regions of increasing Coulomb stress, indicating that they were triggered by stress transfer. Our study suggests that the nucleation of the Mw 6.4 mainshock can be prominently explained by cascade triggering even though we cannot exclude the possible existence of a minor aseismic slip process at the early stage.

We present an integrated image of the lithosphere of Alaska and its western Chukchi and Bering seas shelves based on joint modeling of potential field data constrained by thermal analysis and seismic data. We also perform 3D forward modelling and inversion of Bouguer anomalies to analyze crustal density heterogeneities. The obtained crustal model shows NW regional thickening (32 to 36 km), with localized trends of thicker crust in the Brooks Range (40 km) and in the Alaska and St. Elias ranges (50 km). Offshore, 28–30 km thick crust is obtained near the Bearing slope break and 36–38 km in the northern Chukchi Shelf. In interior Alaska, the crustal thickness changes abruptly across the Denali fault, from 34-36 to the N to above 30 km to the S, that agrees with the presence of a crustal tectonic buttress guiding block motion W and S to the subduction zone. The average crustal density is 2810 kg∙m-3. Denser crust (2910 kg∙m-3) is found S of the Denali Fault related to the oceanic nature of the Wrangellia Composite Terrane rocks. Offshore, less dense crust (< 2800 kg∙m-3) is found along the basins of the Chukchi and Beaufort shelves. At LAB levels, there is a regional SE–NW trend that coincides with the Pacific plate motion, with a lithospheric root beneath the Brooks Range, Northern Slope, and Chuckchi Sea, that may be a relic of the Chukotka-Artic Alaska microplate. The lithospheric root (> 180 km) agrees with the presence of a boundary of cold, strong lithosphere that deflects the strain to the south. South of the Denali Fault the LAB topography is quite complex. East of 150 °W, below Wrangellia and the eastern side of Chugach terranes, the LAB is much shallower than it is west of this meridian. The NW trending limit separating thinner lithosphere in the East and thicker in the West agrees with the two–tiered slab shape of the subducting Pacific Plate. This research has been funded by the We–Me project (PIE–CSIC–201330E111), AGUR 2017–SGR–847, Alpimed (PIE-CSIC-201530E082), Subtetis (PIE-CSIC-201830E039) and funds from the University of Houston. This is a contribution within the PolarCSIC platform. Ref: M. Torne, I. Jiménez–Munt, J. Vergés, M. Fernàndez, A. Carballo, M. Jadamec. Geophysical Journal International, Volume 220, Issue 1, January 2020, Pages 522–540, https://doi.org/10.1093/gji/ggz424.

Waves in bi-ion plasmas are affected by asymmetry. The kinetic theory of the Maxwellian and Lorentzian/Kappa-distributed bi-ion plasma is ameliorated to incorporate the transfer of orbital angular momentum from the helical electric field to the plasma modes. By operating the Laguerre-Gaussian (LG) function, the perturbed distribution function and helical electric field are decomposed into characteristic axial and azimuthal components. In symmetric bi-ion plasmas, the conventional ion modes/waves are only present if both ions have similar masses and the concentration of the electrons is negligible. An imbalance of the symmetry is considered by the contamination of a small fraction of the heavy immobile ions, which urges the negative ions to become heavier than the positive ions in the bi-ion plasma system. The distinct masses of the positive and negative ions provoke mass-asymmetry in the Kappa-distributed bi-ion plasmas. The signature of the unique acoustic-laden twisted modes in non-Maxwellian asymmetric bi-ion plasma is perceived by the temperature of the lighter positive ions and the dynamics of the heavier negative ion. The deliberated results of Landau damping are displayed for distinct values of the azimuthal wave-number and spectral index, temperature-variation and mass-asymmetry.

There is a substantial gap between the current emissions of greenhouse gases and levels required for achieving the 2 and 1.5 °C temperature targets of the Paris Agreement. Understanding the implications of a temperature overshoot is thus an increasingly relevant research topic. Here we explore the carbon cycle feedbacks over land and ocean in the SSP5-3.4-OS overshoot scenario by using an ensemble of Coupled Model Intercomparison Project 6 Earth system models. Models show that after the CO2 concentration and air temperature peaks, land and ocean are decreasing carbon sinks from the 2040s and become sources for a limited time in the 22nd century. The decrease in the carbon uptake precedes the CO2 concentration peak. The early peak of ocean uptake stems from its dependency on the atmospheric CO2 growth rate. The early peak of the land uptake occurs due to a larger increase in ecosystem respiration than the increase in gross primary production, as well as due to a concomitant increase in land-use change emissions primarily attributed to the wide implementation of biofuel croplands. The carbon cycle feedback parameters amplify after the CO2 concentration and temperature peaks due to inertia of the Earth system so that land and ocean absorb more carbon per unit change in the atmospheric CO2 change (stronger negative feedback) and lose more carbon per unit temperature change (stronger positive feedback) compared to if the feedbacks stayed unchanged. The increased negative CO2 feedback outperforms the increased positive climate feedback. This feature should be investigated under other scenarios.

We analyse mantle structure under South America in the DETOX-P1 seismic tomography model, a global-scale, multi-frequency inversion of teleseismic P-waves. DETOX-P1 inverts the most extensive data set of broadband, waveform-based traveltime measurements to date, complemented by analyst-picked traveltimes from the ISC-EHB catalogue. The mantle under South America is sampled by ~665,000 cross-correlation traveltimes measured on 529 South American broadband stations and on 5389 stations elsewhere. By their locations, depths, and geometries, we distinguish four high-velocity provinces under South America, interpreted as subducted lithosphere (’slabs’). The deepest (~1800-1200 km depth) and shallowest (<600 km) slab provinces are observed beneath the Andean Cordillera near the continent’s west coast. At intermediate depths (1200-900 km, 900-600 km), two slab provinces are observed farther east, under Brazil, Bolivia and Venezuela, with links to the Caribbean. We interpret the slabs relative to South America’s paleo-position over time, exploring the hypothesis that slabs sank essentially vertically after widening by viscous deformation in the mantle transition zone. The shallowest slab province carries the geometric imprint of the continental margin and represents ocean-beneath-continent subduction during Cenozoic times. The deepest, far-westerly slab complex formed under intra-oceanic trenches during late Jurassic and Cretaceous times, far west of South America’s paleo-position adjoined to Africa. The two intermediate slab complexes record the Cretaceous transition from westward intra-oceanic subduction to eastward subduction beneath South America. This geophysical inference matches geologic records of the transition from Jura-Cretaceous, extensional “intra-arc” basins to basin inversion and onset of the modern Andean arc ~85 Ma.

The Imperial Valley region of Southeastern California has become one of the most productive agricultural regions in the state. It also has the highest rates of childhood asthma in California. Lack of precipitation in the Imperial Valley has caused the water level of the Salton Sea to recede to a record low since its formation in the early 1900s. Previous studies of wind and dust deposition conducted in other regions have shown how reduced precipitation, ground heating, and the diminishing water level in an arid climate pose a risk of exposing previously-sequestered toxic chemicals to open air, adversely affecting lung health. The purpose of this study is to draw historical parallels between the Aral Sea and Salton Sea in the context of geomorphology, ecology, human health, economics, and human migration, to inform an assessment of environmentally related health impacts of those living in the Imperial Valley region. Future droughts and heatwaves are expected to rise in frequency and severity, and may disproportionately affecting those impacted by financial and health disparities. Future research must include the implications of population health in the context of Planetary Health and GeoHealth as a result of the most recent drought and the receding water levels of the Salton Sea.

Because cross-polarized radar returns are highly associated with volume scatter, radar polarimetry returns tend to show strong evidence of wildfire scars and recovery in forest and chaparral. We focus on the polarimetry images from UAVSAR line SanAnd_08525, which covers a roughly 20 km wide swath over the Transverse Range including parts of the Santa Monica, San Gabriel and San Bernardino Mountains. We select images from four acquisition dates from October 2009 to September 2020, very roughly four years apart. These are compared to fire perimeters from the national GeoMAC and NIFC databases for years 2003-2020, which shows the areas affected by the major fires (west to east) Springs2013, Woolsey2018, Topanga2005, LaTuna2017, Station2009, BlueCut2016, Pilot2016, Slide2007, Butler2007 and many smaller fires. UAVSAR polarimetry images are shown to be helpful in identifying types and boundaries of fire, fifty-meter scale details of vegetation loss, and variability of vegetation recovery in post-fire years.

Hydrogen isotope ratios of sedimentary leaf waxes (δ2HWax values) are increasingly used to reconstruct past hydroclimate. Here, we add δ2HWax values from 19 lakes and four swamps on 15 tropical Pacific islands to an updated global compilation of published data from surface sediments and soils. Globally, there is a strong positive linear correlation between δ2H values of mean annual precipitation (δ2HP values) and the leaf waxes n-C29-alkane (R2 = 0.74, n = 665) and n-C28-acid (R2 = 0.74, n = 242). Tropical Pacific δ2HWax values fall within the predicted range of values based on the global calibration, and the largest residuals from the global regression line are no greater than those observed elsewhere, despite large uncertainties in δ2HP values at some Pacific sites. However, tropical Pacific δ2HWax values in isolation are not correlated with estimated δ2HP values from isoscapes or from isotope-enabled general circulation models. Palynological analyses from these same Pacific sediment samples suggest no systematic relationship between any particular type of pollen distribution and deviations from the global calibration line. Rather, the poor correlations observed in the tropical Pacific are likely a function of the small range of δ2HP values relative to the typical residuals around the global calibration line. Our results suggest that δ2HWax values are currently most suitable for use in detecting large changes in precipitation in the tropical Pacific and elsewhere, but that ample room for improving this threshold exits in both improved understanding of δ2H variability in plants, as well as in precipitation.

We present the first global ocean-biogeochemistry model that uses a telescoping high resolution for an improved representation of coastal carbon dynamics: ICON-Coast. Based on the unstructured triangular grid topology of the model, we globally apply a grid refinement in the land-ocean transition zone to better resolve the complex circulation of shallow shelves and marginal seas as well as ocean-shelf exchange. Moreover, we incorporate tidal currents including bottom drag effects, and extend the parameterizations of the model’s biogeochemistry component to account explicitly for key shelf-specific carbon transformation processes. These comprise sediment resuspension, temperature-dependent remineralization in the water column and sediment, riverine matter fluxes from land including terrestrial organic carbon, and variable sinking speed of aggregated particulate matter. The combination of regional grid refinement and enhanced process representation enables for the first time a seamless incorporation of the global coastal ocean in model-based Earth system research. In particular, ICON-Coast encompasses all coastal areas around the globe within a single, consistent ocean-biogeochemistry model, thus naturally accounting for two-way coupling of ocean-shelf feedback mechanisms at the global scale. The high quality of the model results as well as the efficiency in computational cost and storage requirements proves this strategy a pioneering approach for global high-resolution modeling. We conclude that ICON-Coast represents a new tool to deepen our mechanistic understanding of the role of the land-ocean transition zone in the global carbon cycle, and to narrow related uncertainties in global future projections.

The hydroacoustic monitoring of suspended sediment concentration (SSC) in rivers is based on the inversion of backscatter and attenuation models. To evaluate such models, acoustic backscatter and attenuation were measured from a homogeneous suspension of fine river sediments (clay) in a laboratory tank at various concentrations in the range 1-18~g/l. Agreement between the modelled and measured acoustic backscatter and attenuation values was found to be relatively poor. The results are highly sensitive to particle size and shape which come with large measurement uncertainties and they can be significantly improved by adjusting plausible particle parameters. Various inversion methods combining single or multiple frequencies, analysis of backscatter and/or attenuation, spherical or oblate shape hypothesis for particles and fixed or estimated lognormal grain size distribution are tested. The most promising inversion methods using both backscatter and attenuation information led to accurate SSC estimates.

Firstly, we re-tune an algorithm based on empirical orthogonal functions (EOF) for globally retrieving the chlorophyll a concentration (Chl-a) of phytoplankton functional types (PFTs) from multi-sensor merged ocean color (OC) products. The re-tuned algorithm, namely EOF-SST hybrid algorithm, is improved by: (i) using 30% more matchups between the updated global in situ pigment database and satellite remote sensing reflectance (Rrs) products, and (ii) including sea surface temperature (SST) as an additional input parameter. In addition to the Chl-a of the six PFTs (diatoms, haptophytes, dinoflagellates, green algae, prokaryotes and Prochlorococcus), the fractions of prokaryotes and Prochlorococcus Chl-a to total Chl-a (TChl-a), are also retrieved by the EOF-SST hybrid algorithm. Matchup data are further separated for low and high temperature regimes based on different PFT dependences on SST, to establish the SST-separated hybrid algorithms which further shows improved performance as compared to the EOF-SST hybrid algorithm. The per-pixel uncertainty of the retrieved TChl-a and PFT products is estimated by taking into account the uncertainties from both input data and model parameters through Monte Carlo simulations and analytical error propagation. The uncertainty assessment provided within this study sets the ground to extend the long-term continuous satellite observations of global PFT products by transferring the algorithm and its method to determine uncertainties to similar OC products until today. Satellite PFT uncertainty is also essential to evaluate and improve coupled ecosystem-ocean models which simulate PFTs, and furthermore can be used to directly improve these models via data assimilation.

This study introduces the results from fitting a Bayesian hierarchical spatiotemporal model to COVID-19 cases and deaths at the county-level in the United States for the year 2020. Two models were created, one for cases and one for deaths, utilizing a scaled Besag, York, Mollié model with Type I spatial-temporal interaction. Each model accounts for 16 social vulnerability variables and 7 environmental measurements as fixed effects. The spatial structure of COVID-19 infections is heavily focused in the southern U.S. and the states of Indiana, Iowa, and New Mexico. The spatial structure of COVID-19 deaths covers less of the same area but also encompasses a cluster in the Northeast. The spatiotemporal trend of the pandemic in the U.S. illustrates a shift out of many of the major metropolitan areas into the U.S. Southeast and Southwest during the summer months and into the upper Midwest beginning in autumn. Analysis of the major social vulnerability predictors of COVID-19 infection and death found that counties with higher percentages of those not having a high school diploma and having minority status to be significant. Age 65 and over was a significant factor in deaths but not in cases. Among the environmental variables, above ground level (AGL) temperature had the strongest effect on relative risk to both cases and deaths. Hot and cold spots of COVID-19 cases and deaths derived from the convolutional spatial effect show that areas with a high probability of above average relative risk have significantly higher SVI composite scores. Hot and cold spot analysis utilizing the spatiotemporal interaction term exemplifies a more complex relationship between social vulnerability, environmental measurements, and cases/deaths.

Nearly isotropic comets with very long orbital period are supposed to come from the Oort Cloud. Recent observational and theoretical studies have greatly revealed the dynamical nature of this cloud and its evolutionary history. However, many issues are yet to be known. Our goal is to understand current structure of this cloud as well as its dynamical origin. For estimating the current structure of the Oort Cloud, key information lies in the original orbit of the Oort Cloud new comets (OCNCs) that are defined at a distance where these objects do not receive gravitational perturbation from major planets (such as at rg = 250 au from the Sun before comets enter into the planetary region). There have been several attempts to obtain OCNC’s original orbits, but it never has been an easy task. This requires numerical orbit propagation of the observed comets with high accuracy including perturbation from major disturbing bodies. In addition, non-gravitational forces often play significant roles here. First and foremost, the orbit determination of OCNC includes substantially large uncertainty because of limited number of observational arcs and very large eccentricity of the comets (~1). Here we show our preliminary result of comparison of various catalogues of OCNCs’ original orbital elements at rg = 250 au: So-called the Warsaw catalogues by Krolikowska, the ephemeris given by MPC (Minor Planet Center), that given by Horizons/JPL, and others calculated by a few individuals (Marsden, Kinoshita, and Nakano). The resulting orbits that these catalogues yield are overall similar, but sometimes they are starkly different by reasons yet to be known. Through a series of plots with a help of our own orbit propagation using numerical and analytic methods, we give considerations on which catalogue yields the information that is the most significant (or the most fundamental) for understanding structure, origin, and evolution of the Oort Cloud.

Water mass transformation (WMT) in the North Atlantic plays a key role in driving the Atlantic Meridional Overturning Circulation (AMOC) and its variability. Here, we analyze subpolar North Atlantic WMT in high- and low-resolution versions of the Community Earth System Model version 1 (CESM1) and investigate whether differences in resolution and climatological WMT impact low-frequency AMOC variability and the atmospheric response to this variability. We find that high-resolution simulations reproduce the WMT found in a reanalysis-forced high-resolution ocean simulation more accurately than low-resolution simulations. We also find that the low-resolution simulations, including one forced with the same atmospheric reanalysis data, have larger biases in surface heat fluxes, sea-surface temperatures, and salinities compared to the high-resolution simulations. Despite these major climatological differences, the mechanisms of low-frequency AMOC variability are similar in the high- and low-resolution versions of CESM1. The Labrador Sea WMT plays a major role in driving AMOC variability, and a similar North Atlantic Oscillation-like sea-level pressure pattern leads AMOC changes. However, the high-resolution simulation shows a pronounced atmospheric response to the AMOC variability not found in the low-resolution version. The consistent role of Labrador Sea WMT in low-frequency AMOC variability across high- and low-resolution coupled simulations, including a simulation which accurately reproduces the WMT found in an atmospheric-reanalysis-forced high-resolution ocean simulation, suggests that the mechanisms may be similar in nature.

Indian plate passed over four plumes during its way towards Eurasian plate. These interactions with plumes certainly affected the upper mantle and crust of Indian plate as manifested by its thinned lithosphere and relatively low shear velocities both in crust and upper mantle, but the depth extent of these effects of plume-lithosphere interaction remains ambiguous. In this study, we investigate the mantle transition zone beneath Indian shield using P-to-S receiver functions computed at 24 stations covering the entirety of the Indian shield to investigate the depth extent of the imprints of the plume-lithosphere interaction as well as to study the lateral variations of transition zone beneath a stable intraplate setting like the Indian shield. Our results show good agreement with the results of previous studies as well as with the tomographic models in terms of the average apparent depths of the 410 and 660 discontinuities and the transition zone thickness. However, unlike previous studies, we find a compelling evidence of a persistent mid transition zone discontinuity beneath all the stations and a low velocity layer beneath some regions. We also investigated the frequency dependence of amplitudes of receiver functions and found most of the stations showed strong dependence of amplitudes on frequency. Based on the evidence from our investigation, we demarcate the regions potentially containing relatively more weight percentage of water inside the otherwise considered ‘dry’ mantle transition zone. These regions should be further investigated in detail by a dense seismic network and a realistic 3-D velocity model.

The strength of the biological soft tissue pump in the ocean critically depends on how much organic carbon is produced via photosynthesis and how efficiently the carbon is transferred to the ocean interior. For a given amount of limiting nutrient, phosphate, soft tissue pump would be strengthened if the carbon (C) to phosphorus (P) ratio of sinking organic matter increases as the remineralization length scale of C increases. Here, we present a new data compilation of particle flux stoichiometry and show that C:P of sinking particulate organic matter (POM) in the ocean twilight zone on average is likely to be higher than the C:P ratio of surface suspended POM. We further demonstrate using a physics-biology coupled global ocean model combined with a theory from first principles that an increase in C:P export flux ratio in the ocean’s twilight zone can lead to a considerable drawdown of atmospheric pCO2.

Arctic Ocean freshwater storage increased since the mid 1990s, but the cause was unknown. Now a recent paper by Jahn & Laiho [2020, 2020GL088854] uses ensemble runs of a coupled climate model to suggest that the observed increase is anthropogenic. The paper quantifies when the anthropogenic signals should emerge from the noise of natural variability. This result contextualizes research on the Arctic Ocean freshwater system and sketches an unprecedented opportunity. Future work should elucidate mechanisms, seek to observe the anthropogenic freshwater changes, and investigate the impacts on biogeochemistry and the North Atlantic Ocean circulation.